Mice evolve better, not bigger, balls in sperm race

SIZE isn’t everything. Big testes are usually a telltale sign of strong sexual competition between males. But it seems mice can quickly evolve testes that produce more sperm without growing larger.

When many males mate with a female, their sperm compete with each other to fertilise her eggs. Having larger testes allows males to produce more sperm as they bid to pass on their genes. “There is often a raffle element to fertilisation,” says Stuart Wigby at the University of Oxford. “If you buy more tickets, you’re more likely to win.”

There is a raffle element to fertilisation. If you buy more tickets, you’re more likely to win

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Renée Firman at the University of Western Australia in Perth and her colleagues had previously found that, when mice evolve with high sperm competition, the males produce more sperm – but without developing bigger testes. “We were wondering how the mice had increased their sperm production in the absence of a change in testes size,” says Firman.

Studies by researchers like Stefan Lüpold of Syracuse University in New York and his colleagues had suggested that species under intense sperm competition have more sperm-producing tissue. But these studies only showed a correlation between sperm competition and the density of sperm-producing tissue, and couldn’t prove that one leads to the other.

To test what was happening in mice, Firman’s team allowed them to evolve in two different mating systems&colon; a monogamous system in which males did not have to compete for females, and a polygamous one, in which males all mated with the same group of females. Just 24 generations later, testes from polygamous males contained more sperm-producing tissue than those of monogamous males (Evolution, doi.org/znh).

Lüpold says this is the clearest evidence so far that the level of sperm competition can affect the architecture of testes, and do so in such a short time. “This shows that size isn’t everything,” says Wigby. He points out that blue whales have bigger brains than humans, but aren’t more intelligent.

The findings are unlikely to overturn our understanding of sperm competition and testes size, Wigby says. “Overall, you’d still expect bigger testes in species or populations with much more sperm competition.”

This article appeared in print under the headline “Forget size – it’s what’s inside that counts”